Having health insurance shields you from the full brunt of medical expenses, but it's no guarantee that your costs will be low.

Nevertheless, health-care bills still come as a surprise to many individuals who have insurance.

Close to half of adults who bought their own coverage, along with 43 percent of those with insurance through their employer, said they were surprised by the high cost of their most recent health-care bill in the last year, according to a survey from Bankrate.com.

The personal finance site polled 1,000 individuals in July.

"For a lot of people who've had insurance for decades, there's a creeping sensation that more cost is being thrown onto them," said Taylor Tepper, an analyst at Bankrate.com.

Here's why you can still pay massive bills despite having insurance coverage.

Cost sharing

undefined undefined | iStock | Getty Images

iv line for fluid for patient lying on the bed admitted in hospital

You're not just seeing things: Consumers are paying more money in the form of higher premiums, deductibles and additional expenses.

Last year, annual family premiums for health insurance through the workplace rose to $18,764, an increase of 3 percent from 2016, according to the Kaiser Family Foundation.

Of that amount, on average, workers contributed $5,714 to these family plans, accounting for their share of the premium, Kaiser found.

High deductibles

Though low-premium insurance options are available in the form of high-deductible health plans, consumers are paying a sizeable amount of expenses before their coverage kicks in.

That's because insurance companies and plans are using other levers to share expenses with employees, namely coinsurance, which takes effect after employees hit their annual deductible.

At that point, the insurer will cover a percentage of the cost of service, while the employee pays the rest.

On average, workers were responsible for 19 percent of the cost of a hospital admission in 2017, according to the Kaiser Family Foundation. In comparison, the average copayment for a hospital visit is $336 per admission.

Protect your wallet

Be on your guard for surprise expenses when you're using your coverage. "You have to be an active participant to figure out how to lower your costs," said Tepper of Bankrate.com.

Here's where you can begin:

Understand your plan: Know your deductible, your coinsurance and your out-of-pocket maximum. Be sure to know which of your providers are in-network, as you'll likely pay even more for out-of-network providers.

Use preventive services: Detect health problems before they become complicated and costly. "Figuring out the preventive services offered by your plan is a way to get good quality care while keeping costs as low as possible," Tepper said.

Contest unusually high bills: Resist the urge to just write a check for surprise bills. Call your medical provider and get in contact with your insurer first.

Hire a claims expert to be your advocate if you enter a dispute with your insurer.

"If the bill doesn't look right, don't just pay it," said Tepper. "Start negotiating and see where it leads you."